Cylindrically-shaped balloon-expandable stent

ABSTRACT

This invention is directed to an intracorporeal device formed of a high strength Co—Ni—Cr alloy and is particularly suitable for forming a composite product with a pseudoelastic member formed of NiTi alloy. Suitable intracorporeal products include guidewires and stents. The high strength alloy consists essentially of about 28 to about 65% cobalt, about 2 to about 40% nickel, about 5 to about 35% chromium, up to about 12% molybdenum, up to about 20% tungsten, up to about 20% iron and the balance inconsequential amounts of impurities and other alloying constituents, with a preferred alloy composition including about 30 to about 45% cobalt, about 25 to about 37% nickel, about 15 to about 25% chromium and about 5 to about 15% molybdenum. Intravascular devices such as guidewires, stents and the like can be formed of this high strength Co—Ni—Cr alloy.

RELATED APPLICATIONS

This is a continuation application of copending application Ser. No.8/829,465 filed on Mar. 28, 1991, now U.S. Pat. No. 6,482,166 which is acontinuation of Ser. No. 08/280,209 filed on Jul. 25, 1994, now U.S.Pat. No. 5,636,641.

BACKGROUND OF THE INVENTION

This invention relates to the field of intracorporeal medical devices,and more particularly to elongated intravascular members such asguidewires for percutaneous transluminal coronary angioplasty (PTCA) andstents for maintaining body lumen patency after the lumen has beendilated with a balloon.

In PTCA procedures a guiding catheter is percutaneously introduced intothe cardiovascular system of a patient in a conventional Seldigertechnique and advanced therein until the distal tip of the guidingcatheter is seated in the ostium of a desired coronary artery. Aguidewire is positioned within an inner lumen of a dialatation catheterand then both the catheter and guidewire are advanced through theguiding catheter to the distal end thereof. The guidewire is firstadvanced out of the distal end of the guiding catheter into thepatient's coronary vasculature until the distal end of the guidewirecrosses a lesion to be dilated, then the dilatation catheter having aninflatable balloon on the distal portion thereof is advanced into thepatient's coronary anatomy over the previously introduced guidewireuntil the balloon is properly positioned across the lesion. Once inposition across the lesion, the balloon is inflated one or more times toa predetermined size with radiopaque liquid to dilate the stenosis. Theballoon is then deflated so that blood flow will resume through thedilated artery and the dilatation catheter and the guidewire can beremoved therefrom.

Conventional guidewires for angioplasty and other vascular proceduresusually comprise an elongated core member with one or more taperedsections hear its distal end and a flexible body such as a helical coildisposed about a distal portion of the core member. A shapeable member,which may be the distal extremity of the core member or a separateshaping ribbon such as described in U.S. Pat. No. 5,135,503, herebyincorporated into this application by reference, extends through theflexible body and is secured to a rounded plug at the distal end of theflexible body. Torquing means are provided on the proximal end of thecore member to rotate, and thereby steer, the guidewire while it isbeing advanced through a patient's vascular system. The core member istypically formed of stainless steel, although core member formed ofpseudoelastic NiTi alloys are described in the literature and have beenused to a limited extent in clinical applications.

Further details of guidewires, and devices associated therewith forangioplasty procedures can be found in U.S. Pat. No. 4,516,972 (Samson);U.S. Pat. No. 4,538,622 (Samson et al); U.S. Pat. No. 4,554,929 (Samsonet al.); and copending application Ser. No. 07/994,679 (Abrams et al.)which are incorporated into this application by reference.

Steerable dilatation catheters with fixed, built-in guidewires orguiding members, such as described in U.S. Pat. No. 4,582,181 (now Re33,166) are frequently used because they have better pushability thanover-the-wire dilatation catheters where the guidewires are slidablydisposed within the guidewire lumens of the catheters.

A major requirement for guidewires and other guiding members is thatthey have sufficient column strength to be pushed through a patient'svascular system or other body lumen without kinking. However, they mustalso be flexible enough to avoid damaging the blood vessel or other bodylumen through which they are advanced. Efforts have been made to improveboth the strength and flexibility of guidewires in order to make themmore suitable for their intended uses, but these two properties can bediametrically opposed to one another in that an increase in one usuallyinvolves a decrease in the other. Efforts to combine a separaterelatively stiff proximal section with a relatively flexible distalsection frequently result in an abrupt transition at the junction of theproximal and distal section due to material differences.

What has been needed and heretofore unavailable is an elongatedintravascular body, such as a guidewire, a stent or the like, whichexhibits much higher strength coupled with good ductility than materialscurrently used to form these types of intravascular devices.

SUMMARY OF THE INVENTION

The present invention is directed to a high strength alloy containingcobalt, nickel, and chromium and particularly to a composite producthaving a portion formed of the high strength cobalt-nickel-chromiumalloy and a portion formed of pseudoelastic alloy such as NiTi alloy.

The product of one embodiment of the invention is an elongated memberconfigured for advancement within a body of lumen and is formed at leastin part, of high strength alloy comprising about 28 to about 65% cobalt,about 2 to about 40% nickel, about 5 to about 35% chromium and up toabout 12% molybdenum. Other alloying components include up to about 20%tungsten, up to about 20% iron and up to about 3% manganese. The alloymay also contain inconsequential amounts of other alloying constituents,as well as impurities, typically less than 0.5% each. A presentlypreferred alloy composition for use in the intracorporeal productconsists essentially of about 30 to about 45% cobalt, about 25 to about37% nickel, about 15 to about 25% chromium and about 5 to about 15%molybdenum. As used herein all references to percent composition areweight percent unless otherwise noted. The high strength alloy hasultimate strengths up to and exceeding 300 ksi.

Preferably, the intracorporeal product is formed by first cold workingthe high strength alloy at least 40% of its original transversecross-sectional area in a plurality of cold working stages with the coldworked product being intermediate annealed between cold working stagesat a temperature between about 600° and 1200° C. Those alloys containingmolydenum are age hardenable after cold working and annealing at atemperature between about 400° and about 700° C. For optimum tensilestrength properties the aging is conducted at about 550° to about 680°C., particularly when the high strength alloy is combined with otheralloys as described hereinafter.

In another embodiment of the invention, the cobalt-nickel-chromium alloyis formed into a composite structure with an NiTi alloy which containsabout 25 to about 47% titanium and the balance nickel and up to 10% ofone or more additional alloying elements. Such other alloying elementsmay be selected from the group consisting of up to 3% each of iron,cobalt, platinum, palladium and, chromium and up to about 10% copper andvanadium. This alloy preferably has a stable austenite phase at bodytemperature (about 37° C.) and exhibits pseudoelasticity with a stressedinduced transformation of the austenite phase to a martensite phase atbody temperature at a stress level well about 50 ksi, preferably above70 ksi and in many cases above about 90 ksi. The stress levels causingthe complete stress-induced transformation of the austenite phase to themartensite phase results in a strain in the specimen of at least about4%, preferably over 5%. The region of phase transformation resultingfrom stress preferably begins when the specimen has been strained about1 to 2% at the onset of the phase change from austenite to martensiteand extends to about 7 to about 9% strain at the completion of the phasechange. The stress and strain referred to herein is measured by tensiletesting. Other methods for determining the stress-strain relationship,e.g., applying a bending moment to a cantilevered specimen, provide adifferent relationship from the relationship determined by tensiletesting, because the stresses which occur in the specimen during bendingare not as uniform as they are in tensile testing. The rate of change instress during the phase transformation is considerably less than therate of change thereof either before or after the stress-inducedtransformation. The stress level is relatively constant within thetransformation period.

To form the elongated pseudoelastic NiTi member, the alloy material isfirst cold worked in a plurality of stages, preferably by drawing, toeffect a size reduction of at least about 30% and up to about 70% ormore in the original transverse cross section thereof with intermediateannealing between the cold working stages at temperatures between about600° to about 800° C. for about 5 to about 30 minutes. After the finalcold working stage the cold worked product is given a final anneal at atemperature of about 700° C. to generate final properties. Preferably,the cold worked NiTi alloy product is subjected to tension during thefinal annealing and/or provided with a mechanical straightening followedby thermal treatment to help develop a straight memory. The ultimatetensile strength of the material is well above 200 ksi with an ultimateelongation at failure of about 15%.

In one aspect of the invention the cobalt-nickel-chromium containingalloy and another alloy such as the NiTi alloy described above are coldworked together into a composite product, with both alloys beingsubjected to the same thermomechanical processing to develop a desirablecombination of properties. In particular, a presently preferredthermomechanical processing includes a plurality of drawing steps with areduction of at least 25% in each cold working stage. The cold workedproduct is intermediate annealed between cold working stages at atemperature of about 600° C. and 900° C., e.g., about 750° C. with atime at temperature of about 10 to about 15 minutes. The amount of coldwork in the last working stage should be at least about 50% and can beas high as 95% or more. However, the actual cold working in the finalworking stage is usually determined by the elongation or ductilitydesired in the final product after straightening and aging.

In the above embodiment the elongated Ni—Ti alloy product is an innermember disposed within the inner lumen of an elongated sheath formed ofa Co—Ni—Cr—Mo alloy with an appropriate lubricant and then the assembledunit is processed in a series of size reduction steps involving drawing,or other cold working, followed by an intermediate annealing asdescribed above. The annealing may be performed in line with thedrawing. The Co—Ni—Cr—Mo alloy sheath and the NiTi alloy inner membershould be recrystallization annealed prior to assembly and cold work toprovide maximum ductility by maintaining an equiaxed grain structure andminimum grain growth. After the final cold working step, the compositeproduct is heat treated at a temperature between about 500° and 700° C.and preferably between about 550° and 675° C. for about one minute toabout four hours to age harden the cladding and provide pseudoelasticcharacteristics to the inner member. Tension may be applied during theaging treatment to straighten the product while it is being aged and toprovide straight memory to the Ni—Ti alloy portion of the composite. Forcomposite products with an inner member formed of alloys other thanNi—Ti alloys, the aging conditions, i.e. the temperature and the time attemperature, may be different than that described above for NiTi alloys.

In an alternative embodiment, the NiTi alloy product and the Co—Ni—Cralloy product can be first prepared separately to their desired finalproperties and then combined together by suitable means to form thecomposite product. For example, after final processing, the Co—Ni—Cralloy sheath can be heated to expand the inner lumen therein so that anNiTi inner member can be readily inserted therein. After insertion ofthe NiTi inner member into the inner lumen of the sheath, the latter canbe cooled so that it shrink fits about the NiTi inner member.Alternatively, the NiTi inner member can be inserted into the sheathafter processing while the sheath is still at elevated temperatures andthen cooled to contract the sheath onto the NiTi inner member. Othermeans for combining the NiTi product and the Co—Ni—Cr product includesthe use of an adhesive bond therebetween or a physical connection suchas a set screw extending through the sheath into the core member or someother type of mechanical connection. A wide variety of other means forjoining the Ni—Ti product and the Co—Ni—Cr are contemplated and willbecome apparent to those skilled in the art.

The products of the invention exhibit a very high level of tensilestrength, yet they have excellent ductility. The age hardenedCo—Ni—Cr—Mo alloy can have ultimate tensile strengths above 300 ksi andthe NiTi alloys can have ultimate tensile strengths exceeding 200 ksi.These products are biocompatable and are particularly useful in medicaldevices, which are to be utilized intracorporeally, such as guidewires,stents and the like.

One embodiment of the present invention is directed to an expandablestent which is relatively flexible along its longitudinal axis tofacilitate delivery through tortuous body lumens, but which is stiff andstable enough radially in an expanded condition to maintain the patencyof a body lumen such as an artery when implanted therein.

The stent of the invention generally includes a plurality of radiallyexpandable cylindrical elements which are relatively independent intheir ability to expand and to flex relative to one another. Theindividual radially expandable cylindrical elements of the stent aredimensioned so as to be longitudinally shorter than their own diameters.Interconnecting elements or struts extending between adjacentcylindrical elements provide increased stability and are preferablypositioned to prevent warping of the stent upon the extension thereof.The resulting stent structure is a series of radially expandablecylindrical elements which are spaced longitudinally close enough sothat small dissections in the wall of a body lumen may be pressed backinto position against the luminal wall, but not so close as tocompromise the longitudinal flexibility of the stent. The individualcylindrical elements may rotate slightly relative to adjacentcylindrical elements without significant deformation, cumulativelygiving a stent which is flexible along its length and about itslongitudinal axis but which is still very stiff in the radial directionin order to resist collapse.

The stent embodying features of the invention can be readily deliveredto the desired lumenal location by mounting it on an expandable memberof a delivery, catheter, for example a balloon, and passing thecatheter-stent assembly through the body lumen to the implantation site.A variety of means for securing the stent to the expandable member onthe catheter for delivery to the desired location are available. It ispresently preferred to compress the stent onto the balloon. Other meansto secure the stent to the balloon include providing ridges or collarson the inflatable member to restrain lateral movement, or usingbioresorbable temporary adhesives.

The presently preferred structure for the expandable cylindricalelements which form the stents of the present invention generally have acircumferential undulating pattern, e.g. serpentine. The transversecross-section of the undulating component of the cylindrical element isrelatively small and preferably has an aspect ratio of about two to oneto about 0.5 to one (e.g., the ratio of the height to the width of anundulation). A one to one aspect ratio has been found particularlysuitable. The open reticulated structure of the stent allows for theperfusion of blood over a large portion of the arterial wall which canimprove the healing and repair of a damaged arterial lining.

The radial expansion of the expandable cylinder deforms the undulatingpattern thereof similar to changes in a waveform which result fromdecreasing the waveform's amplitude and the frequency. Preferably, theundulating patterns of the individual cylindrical structures are inphase with each other in order to prevent the contraction of the stentalong its length when it is expanded. The cylindrical structures of thestent are plastically deformed when expanded (except with NiTi alloys)so that the stent will remain in the expanded condition, and, therefore,they must be sufficiently rigid when expanded to prevent the collapsethereof in use. With superelastic NiTi alloys, the expansion occurs whenthe stress of compression is removed so as to allow the phasetransformation from austenite back to martensite and as a result theexpansion of the stent.

The elongated elements or members which interconnect adjacentcylindrical elements should have a transverse cross-section similar tothe transverse dimensions of the undulating components of the expandablecylindrical elements. The interconnecting elements may be formed in aunitary structure with the expandable cylindrical elements from the sameintermediate product, such as a tubular element, or they may be formedindependently and connected by suitable means, such as by welding or bymechanically securing the ends of the interconnecting elements to theends of the expandable cylindrical elements. Preferably, all of theinterconnecting elements of a stent are joined at either the peaks orthe valleys of the undulating structure of the cylindrical elementswhich form the stent. In this manner there is no shortening of the stentupon expansion, when measured from the outermost ends of theinterconnecting members connected to the cylindrical elements atopposite ends of the stent.

The number and location of elements interconnecting adjacent cylindricalelements can be varied in order to develop the desired longitudinalflexibility in the stent structure both in the unexpanded as well as theexpanded condition. These properties are important to minimizealteration of the natural physiology of the body lumen into which thestent is implanted and to maintain the compliance of the body lumenwhich is internally supported by the stent. Generally, the greater thelongitudinal flexibility of the stent, the easier and more safely it canbe delivered to the implantation site.

In a presently preferred embodiment of the invention the stent isconveniently and easily formed by coating stainless steel hypotubingwith a material resistant to chemical etching, and then removingportions of the coating to expose portions of underlying tubing whichare to be removed to develop the desired stent structure. The exposedportions of the tubing are removed by chemically etching from the tubingexterior leaving the coated portion of the tubing material in thedesired pattern of the stent structure. The etching process developssmooth openings in the tubing wall without burrs or other artifactswhich are characteristic of mechanical or laser machining processes inthe small sized products contemplated. Moreover, a computer controlledlaser patterning process to remove the chemical resistive coating makesphotolithography technology adaptable to the manufacture of these smallproducts. The forming of a mask in the extremely small sizes needed tomake the small stents of the invention would be a most difficult task. Aplurality of stents can be formed from one length of hypotubing byrepeating the stent pattern and providing small webs or tabs tointerconnect the stents. After the etching process, the stents can beseparated by severing the small webs or tabs which connect them. Thestents may further be electrochemically polished in an aqueous solutionand treated, if desired by applying a biocompatible coating. These andother advantages of the invention will become more apparent from thefollowing detailed description of the invention when taken inconjunction with the accompanying exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view, partially in section, of a guidewireembodying features of the invention.

FIG. 2 is an elevational view, partially in section, of an alternativeguidewire structure embodying features of the invention.

FIG. 3 is an elevational view, partially in section of a stent beingexpanded by an inflatable balloon on an intravascular catheter within astenosed region of a patient's artery.

FIG. 4 is a perspective view of an intravascular stent which may beformed of the alloy composition of the invention.

FIG. 5 is an elevational view, partially in section, of a stentembodying features of the invention which is mounted on a deliverycatheter and disposed within a damaged artery.

FIG. 6 is an elevational view, partially in section, similar to thatshown in FIG. 1 wherein the stent is expanded within a damaged artery,pressing the damaged lining against the arterial wall.

FIG. 7 is an elevational view, partially in section showing the expandedstent within the artery after withdrawal of the delivery catheter.

FIG. 8 is a plan view of a flattened section of a stent of the inventionwhich illustrates the undulating pattern of the stent shown in FIG. 4.

FIGS. 9 through 12 are perspective views schematically illustratingvarious configurations of interconnective element placement between theradially expandable cylindrical elements of the stent.

FIG. 13 is a plan view of a flattened section of a stent illustrating analternate undulating pattern in the expandable cylindrical elements of astent out of phase.

FIG. 14 is a schematic representation of equipment for selectivelyremoving coating applied to tubing in the manufacturing of the stents ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a guidewire 10 which includes a core member 11 havingan outer sheath 12 formed of a Co—Ni—Cr alloy and an inner member 13formed of Ni—Ti, a helical coil 14 on the distal end of the core memberwith a shaping ribbon 15% extending between the distal end of the coremember and a rounded plug 16 which connects the distal end of theshaping ribbon with the distal end of the helical coil. The distalsection 17 of the core member 11, which is disposed primarily within thecoil 14, is tapered to sequentially smaller diameters to providegradually increasing flexibility along the length of the distal portionof the guidewire 10. The taper is formed by removing the sheath 12formed of high strength Co—Ni—Cr alloy which exposes the inner NiTialloy member 13 having moderate strength and substantial flexibilitywhich may then be ground in a conventional manner to one or more smallerdiameter sections.

FIG. 2 depicts a guidewire 30 with a construction wherein the tapereddistal section 31 of the core member 32 extends to the plug 33 whichconnects the distal end of the core member to the distal end of thehelical coil 34 disposed about the distal section of the core member.The proximal section 35 of the core member 32 is of compositeconstruction as in the prior embodiment with a sheath 36 of highstrength Co—Ni—Cr alloy and an inner member 37 of a psuedoelastic NiTialloy. The high strength sheath 36 is removed from the core member toform the tapered distal section 31 to increase the flexibility of thedistal section of the guidewire 30.

The elongated proximal portions of the guidewires are generally about130 to about 140 cm in length with an outer diameter of about 0.006 toabout 0.018 inch for coronary use. Larger diameter guidewires may beemployed in peripheral arteries and other body lumens. The lengths ofthe smaller diameter and tapered sections can range from about 2 toabout 20 cm, depending upon the stiffness or flexibility desired in thefinal product. The helical coil is about 20 to about 45 cm in length,has an outer diameter about the same size as the diameter of theelongated proximal portion, and is made from wire about 0.002 to about0.003 inch in diameter. The shaping ribbon and the flattened distalsection of distal portion have rectangular transverse cross-sectionswhich usually have dimensions of about 0.001 by 0.003 inch. The overalllength of a guidewire is typically about 175 cm.

A presently preferred cobalt-nickel containing alloy is commerciallyavailable as MP35N from Carpenter Technology Corporation which has anominal composition of about 35% cobalt, about 35% nickel, about 20%chromium and about 10% molybdenum. Other commercially available alloysinclude Elgiloy from Elgiloy Limited Partnership and Haynes 188 fromHaynes International.

The following example is given to illustrate the method of forming thecore member of a guidewire in accordance with the invention. A NiTialloy rod having a composition of about 55.9% Ni and 44.1% Ti was drawnto a diameter of about 0.06 inch. The as-drawn wire, which was in a coldworked condition (e.g. 50% cold work), was ground or etched to removetenaceous surface oxides and then annealed at 700° C. for about onehour. A tubular sheath having a nominal composition of 35% Co, 35% Ni,20% Cr and 10% Mo was formed with an outer diameter of about 0.114 inchand an inner diameter of about 0.068 inch in an annealed condition. TheNiTi wire was disposed within the inner lumen of the high strengthsheath and the assembly was drawn in a series of five stages with a 50%reduction in area followed by heat treating at 750° C. for 15 minutes ineach stage. The fifth stage was followed by a sixth stage which includeddrawing with a cold work of about 16% followed by heat treating at 750°C. and a seventh stage which included drawing with a cold work of about50% but with no heat treating. The final cold worked product was aged attemperatures of about 650° for about one minute to develop maximumbending, yield and modulus with minimum spring back.

The composite core member of the invention provides a number offavorable properties and characteristics. The outer sheath of highstrength cobalt-nickel alloy provides the necessary stiffness and pushand the inner NiTi alloy member provides the desirable distalflexibility. Another advantage of the composite product of the presentinvention, when utilized as a core member of a guidewire, is that theproximal end of the flexible coil can be soldered or brazed to theCo—Ni—Cr alloy sheath so as to avoid the problems with soldering thecoil to a NiTi alloy which Is very difficult to bond to by conventionalsoldering techniques because of the tenaceous oxide which usually formson the surfaces of titanium containing alloys.

FIGS. 3 and 4 illustrate another embodiment of the invention wherein thehigh strength Co—Ni—Cr alloy is in the form of an intraluminal stent 40,which as shown in FIG. 3, is expanded by the balloon of 41 of catheter42 within a stenosis 43. After permanent expansion of the stent 40within the body lumen 44, such as a coronary artery, the balloon 41 isdeflated and the catheter 42 withdrawn. The high strength developed bythe Co—Ni—Cr alloy allows the stent to be formed of thinner material,yet provide the radial rigidity to hold the body lumen upon deflation ofthe balloon. The balloon utilized to expand the stent is similar in manyrespects to a dilatation balloon used in angioplasty procedures in thatit is a generally inelastic balloon formed of a suitable polymericmaterial such as a high density polyethylene, polyethylene terephthalateand polyolefin, e.g. Surlyn ®. A particularly suitable stent design isdisclosed in copending application Ser. No. 08/164,986, filed on Dec. 9,1993, which is assigned to the present assignee and which isincorporated herein by reference.

Typically the stent has the same outer diameter as the diameter to thehypotubing from which it is made. The wall thickness of the hypotubingand the transverse dimension of any elongate element of the unexpandedstent formed from the hypotubing would be about 0.003 inches (0.0767mm).

FIG. 4 is an enlarged perspective view of a stent 50 as shown in FIGS. 3and 5-7 with one end of the stent shown in an exploded view toillustrate in greater detail the placement of interconnecting elements53 between adjacent radially expandable cylindrical elements 52. Eachpair of the interconnecting elements 53 on one side of a cylindricalelement 52 are preferably placed to achieve maximum flexibility for astent. In the embodiment shown in FIG. 8 the stent 50 has threeinterconnecting elements 53 between adjacent radially expandablecylindrical elements 52 which are 120 degrees apart. Each pair ofinterconnecting elements 53 on one end of a cylindrical element 52 areoffset radially 60 degrees from the pair on the other side of thecylindrical element. The alternation of the interconnecting elementsresults in a stent which is longitudinally flexible in essentially alldirections. Various configurations for the placement of interconnectingelements are possible, and several examples are illustratedschematically in FIGS. 9-12. However, as previously mentioned, all ofthe interconnecting elements of an individual stent should be secured toeither the peaks or valleys of the undulating structural elements inorder to prevent shortening of the stent during the expansion thereof.

FIG. 5 illustrates a stent 50 incorporating features of the inventionwhich is mounted onto a delivery catheter 51. The stent generallycomprises a plurality of radially expandable cylindrical elements 52disposed generally coaxially and interconnected by elements 53 disposedbetween adjacent cylindrical elements. The delivery catheter 51 has anexpandable portion or balloon 54 for expanding of the stent 50 within anartery 55. The artery 55, as shown in FIG. 5, has a dissected lining 56which has occluded a portion of the arterial passageway.

The delivery catheter 51 onto which the stent 50 is mounted, isessentially the same as a conventional balloon dilatation catheter usedfor angioplasty procedures. The balloon 54 may be formed of suitablematerial such as polyethylene, polyethylene terephthalate, polyvinylchloride, nylon and ionomers such as Surlyn® manufactured by the PolymerProducts Division of the Du Pont Company. Other polymers may also beused. In order for the stent 50 to remain in place on the balloon 54during delivery to the site of the damage within the artery 55, thestent 50 is compressed onto the balloon. A retractable protectivedelivery sleeve 20 as described in copending application Ser. No.07/647,464, filed on Apr. 25, 1990 and entitled STENT DELIVERY SYSTEMmay be provided to further ensure that the stent stays in place on theexpandable portion of the delivery catheter 51 and prevent abrasion ofthe body lumen by the open surface of the stent 50 during delivery tothe desired arterial location. Other means for securing the stent 50onto the balloon 54 may also be used, such as providing collars orridges on the ends of the working portion, i.e., the cylindricalportion, of the balloon.

Each radially expandable cylindrical element 52 of the stent 50 may beindependently expanded. Therefore, the balloon 54 may be provided withan inflated shape other than cylindrical, e.g., tapered, to facilitateimplantation of the stent 50 in a variety of body lumen shapes.

In a preferred embodiment, the delivery of the stent 50 is accomplishedin the following manner. The stent 50 is first mounted onto theinflatable balloon 54 on the distal extremity of the delivery catheter51. The balloon 54 is slightly inflated to secure the stent 50 onto theexterior of the balloon. The cathether-stent assembly is introducedwithin the patient's vasculature in a conventional Seldinger techniquethrough a guiding catheter (not shown). A guidewire 58 is disposedacross the damaged arterial section with the detached or dissectedlining 56 and then the catheter-stent assembly is advanced over aguidewire 58 within the artery 55 until the stent 50 is directly underthe detached lining 56. The balloon 54 of the cathether is expanded,expanding the stent 50 against the artery 55, which is illustrated inFIG. 2. While not shown in the drawing, the artery 55 is preferablyexpanded slightly by the expansion of the stent 50 to seat or otherwisefix the stent 50 to prevent movement. In some circumstances during thetreatment of stenotic portions of an artery, the artery may have to beexpanded considerably in order to facilitate passage of blood or otherfluid therethrough.

The stent 50 serves to hold open the artery 55 after the catheter 51 iswithdrawn, as illustrated by FIG. 7. Due to the formation of the stent50 from an elongated tubular member, the undulating component of thecylindrical elements of the stent 50 is relatively flat in transversecross-section, so that when the stent is expanded, the cylindricalelements are pressed into the wall of the artery 55 and as a result donot interfere with the blood flow through the artery 55. The cylindricalelements 52 of the stent 50 which are pressed into the wall of artery 55will eventually be covered with endothelial cell growth which furtherminimizes blood flow interference. The undulating portion of thecylindrical sections 52 provide good tacking characteristics to preventstent movement within the artery. Furthermore, the closely spacedcylindrical elements 52 at regular intervals provide uniform support forthe wall of artery 55, and consequently are well adapted to tack up andhold in place small flaps or dissections in the wall of the artery 55 asillustrated in FIGS. 6 and 7.

FIG. 12 illustrates a stent of the present invention wherein threeinterconnecting elements 53 are disposed between radially expandablecylindrical elements 52. The interconnecting elements 53 are distributedradially around the circumference of the stent at a 120 degree spacing.Disposing four or more interconnecting elements 53 between adjacentcylindrical elements 52 will generally give rise to the sameconsiderations discussed above for two and three interconnectingelements.

The properties of the stent 50 may also be varied by alteration of theundulating pattern of the cylindrical elements 53. FIG. 13 illustratesan alternative stent structure in which the cylindrical elements are inserpentine patterns but out of phase with adjacent cylindrical elements.The particular pattern and how many undulations per unit of lengtharound the circumference of the cylindrical element 52, or the amplitudeof the undulations, are chosen to fill particular mechanicalrequirements for the stent such as radial stiffness.

The number of undulations may also be varied to accommodate placement ofinterconnecting elements 53, e.g., at the peaks of the undulations oralong the sides of the undulations as shown in FIGS. 8 and 13.

The stent 50 of the present invention can be made in many ways. However,the preferred method of making the stent is to coat a thin-walledtubular member, such as stainless steel hypotubing, with a materialwhich is resistive to chemical etchants, and then to remove portions ofthe coating to expose the underlying hypotubing which is to be removedbut to leave coated portions of the hypotubing in the desired patternfor the stent so that subsequent etching will remove the exposedportions of the metallic tubing, but will leave relatively untouched theportions of the metallic tubing which are to form the stent. The coatedportion of the metallic tube is in the desired shape for the stent. Anetching process avoids the necessity of removing burrs or slag inherentin conventional or laser machining processes. It is preferred to removethe etchant-resistive material by means of a machine-controlled laser asillustrated schematically in FIG. 14. The stent may be electrochemicallypolished in an aqueous solution and treated, if desired, by applying abiocompatible coating.

While the present invention has been described herein in terms ofcertain preferred embodiments, those skilled in the art will recognizethat a variety of modifications and improvements can be made to thepresent invention without departing from the scope thereof.

What is claimed is:
 1. A cylindrically shaped balloon-expandable stentconfigured for use in a coronary artery, comprising: a plurality ofindependently expandable and interconnected cylindrical elements formedof an alloy containing cobalt, chromium, molybdenum, and nickel andgenerally aligned along a common longitudinal axis; and the stent has afirst low profile configuration for delivery and a second radiallyexpandable configuration and is plastically deformable from the firstlow profile delivery configuration to the second radially expandedconfiguration, the second radially expanded configuration having adiameter suitable to hold open the coronary artery, wherein thecylindrical elements of the stent have an clasticity insufficient toallow expansion from the first low profile delivery configuration to thesecond radially expanded configuration without plastic deformation so asto be permanent, and the cylindrical elements having an undulatingcomponent, wherein the undulating component has an electrochemicallypolished metallic surface; the stent further comprising a biocompatiblecoating on the electrochemically polished metallic surface of thecylindrical elements.
 2. The stent of claim 1, wherein the alloycontains about 28 to about 65 weight percent cobalt, about 5 to about 35weight percent chromium, about 2 to about 40 weight percent nickel. 3.The stent of claim 2, wherein the alloy further contains molybdenum upto about 15 weight percent.
 4. The stent of claim 1, wherein the alloyfurther contains molybdenum up to about 15 weight percent.
 5. The stentof claim 1, wherein at least one of the cylindrical elements has anundulating component out of phase with the undulating component of atleast another one of the cylindrical elements.
 6. The stent of claim 1,wherein the cross-section of the undulating component of the cylindricalelement has an aspect ratio of about one to one.
 7. The stent of claim1, wherein the cross-section of the undulating component of thecylindrical element has a height-to-width aspect ratio of abouttwo-to-one.